I don't see where anyone answered this so I will: The carbs for 2F engines have a vacuum actuated secondary, those (2 bbl) for F engines have the mechanical actuated secondary. You can actually feel the secondary engage through your foot on the carbs for F engines. You can not feel it on the ones for 2Fs.

Jim Chenoweth told me that the 2F carbs do flow more CFMs than the F style carbs, and that the later ones used on late FJ40s and all the FJ60s flow more still. He also said that all of the 2F carbs flow more than either Weber 38/38 or Holly 350s.

Thanks Jeff for that description of the actuation of the secondary - my PO has always maintained that they weren't sure that the secondary was kicking in, even when swapping carbs. I found the problem and documented it in this thread:http://risingsun4x4club.org/forum2/s...ad.php?t=16462

I found that my Aisan 38/40 (12/78 build date) has 31/33 venturis. I'm guessing the change to 31/35 venturis came about in '81 or so, with the introduction of the FJ60 model, but would have to check out a few to know for sure.

From my research on MUD, it would seem then that an Aisan 38/38 (F engine) has 28/31 venturis. All very interesting stuff.

I believe a Weber 32/36 is rated for 300 CFM, a Weber 38/38 (with 26/26 venturis) is rated at 375-400 cfm, I *think* the Holley 350 is rated for 350 CFM, and the Aisan 38/40 (for the '75-'76 model year) is rated for 425 CFM:Jim C. on the subject

By the way, a 2F needs at least 300 CFM when turning 4000 rpm (and probably closer to 373 CFM if you take 0.8 volumetric efficiency). If you have a balanced engine that can make use of more air (ported, cam, etc.) and run higher rpms than 4000 you could calculate what you need, could be as high as 466 CFM at 5000 rpm (depending on your V.E.).CFM formula based on engine displacement/rpm

I'm not sure about the other Aisan 38/40 carbs (which Jim says are almost as big) or the Aisan 38/38. I've been trying a little bit to calculate it out, but my numbers aren't quite working out yet. I think they're usually rated by bench testing rather than calculation. Not sure why the '75-'76 would have the highest flow, unless I'm wrong with my guesses of venturi size changes over the years, or unless there's some other factor limiting flow...

gettting ready do this, fj? pighead and i are getting ready to do at least two, maybe his also. he got the build dates and i just have to order the kits now.

many have said the napa kits are more complete than the keyster kits. anyone have an opinion/knowledge on the quality comparison?

It really is a mixed bag, even within the Keyster line. I used a parts store kit for a rebuild on a 22R all the rest have been Keyster kits purchased from Kurt at Cruiseroutfitters. If a part is not damaged, I prefer to clean it up and put it back in versus using a new part from the kit. Typically, I end up using the gaskets and rubber housing for the accelerator pump from the kit and ignoring the rest.

I have had isssue with the rubber cup on new accelterator pumps getting destroyed upon installation and really prefer to use (most often reuse) the ones with the leather.